DO NOT MERGE: WM: Only allow system to use NO_INPUT_CHANNEL.

NO_INPUT_CHANNEL is a hidden WM flag that allows creation of a window
without an input channel. Unfortunately in releases prior to Android R
this would allow creation of a Window which will not be known to the
InputDispatcher at all. This means that the logic generating
FLAG_OBSCURED will work and a window will be able to overlay another
window without the overlayed window being notified. In Android R and
later this isn't a problem as the InputDispatcher is informed of all
windows, input channel or not. For past Android releases, this patch
disables NO_INPUT_CHANNEL for use outside of the WM.

Bug: 152064592
Test: Existing tests pass
Change-Id: I7e1f45cba139eab92e7df88d1e052baba0ae2cc6